The invention pertains to sensors and particularly to resonant sensors. More particularly, the invention pertains to micromechanical sensors having a composite or multi-material resonant microbeam having a frequency that is temperature invariant or, in other terms, has a zero temperature coefficient of frequency (TCF).
Related art micromechanical sensors use resonators fabricated from a single material such as quartz, single crystal silicon or deposited polysilicon films. The resonators of each of these approaches are subject to variations in frequency with temperature due to changes in density, elastic moduli and other parameters. The present invention reduces these variations with a multi-material resonator that has a frequency-to-temperature sensitivity of nearly zero which is comparable with very high precision resonators such as those of well-cut quartz.
The present invention involves a composite resonant microstructure, supported firmly at each end, utilizing a second, dissimilar material so that the shift in frequency with device temperature can be tailored. The specific geometries for this resonator is such that the shift in the resonant frequency with temperature is virtually zero. The composite resonator is made of at least two deposited thin films. One of the films is polysilicon, and the second is from a group including silicon nitride and silicon oxide. The composite thin film resonator may have the second material film centrally located so as to minimize bending stresses on the second material. The second material film may traverse some, but not all, of the length and width of the resonator. The composite thin film resonator has the second film material at a prescribed thickness, and the area of the second material film is adjusted to tailor the temperature sensitivity of the resonator.
The composite thin film resonator may be designed to operate in a balanced mode, where the second material film traverses an area where vibrations are minimal in that the temperature sensitivity of the resonator is tailorable and can be made practically non-existent.
A micromechanical sensor incorporating one or more resonators, with integral vacuum cavities over the resonators, such that the shift in frequency with temperature can also be tailored so as to be zero.
A sensor containing two or more resonators of differing lengths, for separating the frequencies of the individual resonators, wherein the lengths or widths of the second material film are not identical, can still allow the temperature sensitivity of the individual resonators to be tailored and set to zero. Such sensor may be a pressure sensor or an accelerometer containing one or more resonators having a temperature coefficient of frequency that can be reduced to zero. A strain sensor may likewise contain one or more resonators such that the temperature coefficient of frequency can be trimmed and set to zero.